This invention relates to contact lenses and in particular to bifocal and trifocal contact lenses.
Conventional bifocal contact lenses can be divided into two main types, i.e.
1. Concentric bifocals in which the distance vision zone is in the centre and the reading or near vision zone is a peripheral ring or toroid around the central area. Occasionally the zones are reversed. PA1 2. Bifocals which resemble scaled-down spectacle bifocal lenses. These lens comprise two D-shaped segments, the lower segment being the near vision zone.
In the case of the first type, the size of the central zone is critical, particularly if the lens is fitted tightly to the cornea so that little movement occurs on blinking. The amount of light entering the eye from the distant and near vision zones must be approximately equal, otherwise vision will be biased towards either distance or reading. As the pupil diameter is not static but varies according to the brightness of the light, a compromise must be made in selecting the size of the central zone. This problem is made worse by the fact that the difference in pupil size between the maximum and minimum varies from person to person.
Bifocal lenses of the second type generally have to be fitted slightly looser so that the lens can move over the cornea so that when the wearer is looking straight ahead the line dividing the two segments is below the centre of the pupil, while for reading the opposite situation applies. Thus the location of the dividing line between the distance and reading zones is critical for satisfactory fitting of this type of lens. Although variations in pupil size are less important in the case of this type they can affect the result.
For these reasons a large measure of trial and error inevitably occurs in fitting bifocal lenses of both types, and the practitioner therefore requires a very large inventory to cover all the necessary permutations.